Roger H. Glick, P.E., Ph.D. Leila Gosselink, P.E.

Watershed Protection Department City of Austin

Presented at

2013 International SWAT Conference Toulouse, France

Université Paul-Sabatier

Study area

Study Watershed: Tributary to Gilleland Creek

4.99 km2

1233 acres

Elevation Data

Min Elev = 435 ft

Max Elev = 609 ft

Model Sub-basins

Site Slopes

Site Soils

City of Austin Ordinances: Land Use & Controls Undeveloped [UND]

Pre-Waterways Ordinances [Pre-ORD], <1974

• No controls

• Limited creek easements, >320 ac.

Waterway Ordinance [WO], 1974-1986

• Detention only

• Wider easements, >320 ac

Comprehensive Watershed Ordinance [CWO], 1986-present*

• Detention and ½”+ sed-fil

• Creek buffer and water quality transition zone, >320 ac

Watershed Protection Ordinance [WPO], proposed

• Detention and ½”+ sed-fil

• Creek buffer, >64 ac (no WQTZ)

Undeveloped Land Use

Pre-Ord Land Use (<1974)

WO Land Use (~1974-86)

CWO Land Use (1986-present)

WPO Land Use (proposed)

HRU Distribution

Model Scenarios Developed Conditions

Irrigation and fertilizer on lawns and commercial; except high slopes

Increased roughness & conductivity in channels

100% of developed residential & commercial land treated by BMPs; some land uses excluded.

One large detention basin mid-basin (reach 9)

Detention Pond Location

Effects of Ordinances

Impacts on Flooding

Computation of shear 𝜏 = 𝛾𝑤 ∙ 𝐷𝐻 ∙ 𝑆𝑤

where,

τ = shear (Pa)

γw = density of water (kg/m3)

DH = depth of water (m)

Sw = channel slope (m/m)

Impacts on Erosion Potential

Impacts on Erosion Potential

Computation of critical shear 𝜏𝑐 = Θ𝑐(𝑆𝑔 − 1) ∙ 𝛾𝑤 ∙ 𝑑50

where,

τc = critical shear (Pa)

γw = density of water (kg/m3)

Sg = specific gravity of soil, 2.65

d50 = median particle diameter (m)

θc = critical Shield’s parameter, 0.047

ES was defined as:

𝐸𝑆 = ( 𝜏 − 𝜏𝑐) for all τ>τc

Impacts on Erosion Potential

Impacts on Erosion Potential

Impacts on Aquatic Life Changes in hydrology affect aquatic live in two ways

Changes in the wet-dry cycle interrupting species life cycles

Increased variability affecting habitat

Impacts on Aquatic Life

Impacts on Aquatic Life (cont.)

Impacts on Aquatic Life (cont.)

Impacts on Aquatic Life (cont.)

Impacts on Aquatic Life (cont.)

Impacts on Aquatic Life (cont.)

Conclusions Development prior to regulations had negative

impacts on flooding, erosion and aquatic life potential.

Detention designed for large design rainfall events will not address the increased frequency of higher flow rates.

Flood detention alone will not address issues of erosion and aquatic life (and my be detrimental).

Austin regulations since CWO implementation have been beneficial with respect to flooding, erosion and aquatic life potential.